The present invention generally relates to a heat exchanger that lessens the thermal shock to heat exchanger tubes by placing a relatively large thermal mass in front of the core to act as a buffer to reduce high thermal stresses that may be caused by global temperature patterns and localized high gradients that occur in system transients such as start up and shutdown and power transitions.
Heat exchangers that are located in a high temperature environment are subject to high thermal stresses, leading to limited life. This limited life decreases performance via increased leakage and greatly increased life cycle costs of the equipment. Precoolers in ECS (environmental control system) bleed circuits and recuperators placed in a turbine exhaust flow path are examples of applications that experience limitations on heat exchanger life. High thermal stresses may be caused by global temperature patterns and localized high gradients that occur in system transients such as start up and shutdown. Furthermore, in tubular units, large thermal gradients may cause localized buckling that can cause catastrophic failure of the heat exchanger core.
U.S. Pat. No. 3,785,535 discloses a conventional annular heat exchanger with plates graduated in thickness at the ends. Damping of the thermal effect of the incoming exhaust gas flow is achieved by using the graduated plates of varying thickness near the ends of the heat exchanger. The damping effect occurs in the stack-height direction and is localized. The patent does not disclose lessening the thermal impact upon the overall heat exchanger core.
U.S. Pat. No. 4,697,633 discloses a conventional heat exchanger having a thermally balanced restraint system. Tie rods are placed in the fluid stream to force the tie rod temperature to follow the bulk temperature of the core of a plate heat exchanger construction. This is done to reduce the thermal stresses on the heat exchanger and the tie rod as the thermal expansion increases of the core and the tie rods are made more equal. In this configuration, the tie rods restrain the core and act as a supporting structural member that may experience and induce upon the core thermal loads due to being constrained.
As can be seen, there is a need for an improved heat exchanger where high thermal stresses on the heat exchanger core, especially during startup and shutdown, may be avoided, thereby increasing the lifespan of the heat exchanger unit.